Calmodulin-dependent cyclic nucleotide phosphodiesterase has been purified to homogeneity from bovine and sheep brain using a rapid one-day method. The purified enzyme behaves as a dimer in the analytical ultracentrifuge. Fluorescence studies of the interaction between the purified enzyme and dansyl calmodulin indicate that formation of complex produces an increase in the affinity of calmodulin for Ca++. Radiation inactivation studies, in addition to those with gel filtration and electrophoresis, suggest that enzyme monomer is fully active without calmodulin while dimer is inactive without calmodulin, thus providing a physical description for basal and calmodulin-dependent activity.